Oh, yeah, it really is that Solid Silicon. You can make your own speculations from the commit log, though regardless of whether Solid Silicon is truly a separate concern or a Raptor subsidiary, it wouldn't be surprising that Raptor resources are assisting since they've kind of bet the store on the S1.
Timothy Pearson's comments in the Electron Github suggest that Google has been pretty resistant to incorporating support for architectures outside of their core platforms. This is not a wholly unreasonable position on Google's part but it's not a particularly charitable one, and unlike Mozilla, the Chrome team doesn't really have the concept of a tier-3 build nor any motivation to. That kind of behaviour is all the more reason not to encourage browser monocultures because it's not just the layout engine that causes vendor lock-in. Fortunately V8, the JavaScript engine, is maintained separately, and reportedly has been more accommodating presumably because of things like Node.js on IBM hardware (even IBM i via PASE!).
Mozilla is much more accepting of this as long as regressions aren't introduced. This is why TenFourFox patches were largely not upstreamed since they would potentially cause problems with Cocoa widgets in later versions of macOS, though what patches were generally applicable I would do so. The main reason I'm still maintaining the Firefox ppc64le JIT patches outside is because I still can't solve these recent startup crashes deep within Wasm code, which largely limits me to Baseline Compiler and thus is not suitable for loading into the tree yet (we'd have to also upstream pref changes that would adversely affect tier-1 until this is fixed). I still intend to pull these patches up to the next ESR, especially since Github is glacially slow now without a JIT and it's affecting my personal ability to do other tasks. Maybe I should be working on something like rr for ppc64le at the same time because stepping through deeply layered code in gdb is a great way to go stark raving mad.